1. Field of the Invention
The present invention relates to a manufacturing method of a piston for a swash plate type compressor with variable capacity. More particularly, the present invention relates to a manufacturing method of a piston for a swash plate type compressor with variable capacity, which manufactures a piston from two round sections of piston members through friction stir welding without forming a hole in welded portions of the piston members, thereby improving durability of the welded portions and easily manufacturing a piston regardless of the size of the outer diameter of the piston members.
2. Background of the Related Art
In general, a swash plate type compressor with variable capacity of a cooling apparatus of an automobile includes: front and rear housings forming sealed spaces therein, such as a crank chamber, a suction chamber, a discharge chamber, and so on; a cylinder block installed between the front and rear housings and having a number of cylinder bores arranged in the circumferential direction thereof; a driving shaft rotatably mounted on the center of the cylinder block; a lug plate and a swash plate respectively mounted on the driving shaft; a number of pistons coupled along the circumference of the swash plate via shoes; a valve unit disposed between the cylinder block and the rear housing; a control valve for controlling the movement of the pistons; and a spring elastically mounted on the swash plate for supporting the swash plate at the minimum slope angle when the lug plate does not rotate.
In such a swash plate type compressor with variable capacity, weight of the piston acts as an inertia preventing reciprocating motion of the piston during a refrigerant compressing process, thereby deteriorating compression efficiency. To solve the above problem, the piston is manufactured of light-weight material, and recently, the piston has a hollow part formed therein for more light weight.
Conventionally, to manufacture the hollow piston, a first piston member and a second piston member are molded separately, wherein the first piston member has a bridge having a shoe pocket for seating shoes connecting the piston and the swash plate therein, and the second piston member has a hollow part. After that, the first and second piston members are temporarily coupled to each other after mechanical processing, and then, the coupled surfaces of the first and second piston members are welded and adhered to each other by electron beam welding under a vacuum condition.
However, the conventional hollow piston manufacturing method has several problems in that it is necessary to process the coupled surfaces precisely, that it takes too much time to assemble the first and second piston members, and that it is complex and difficult to process the first and second piston members, so that its productivity is deteriorated, and defective rate becomes too high. Furthermore, the conventional hollow piston manufacturing method has another problem in that durability of the piston is lowered as fine holes are formed inside the piston during the electron beam welding step. The conventional hollow piston manufacturing method has a further problem in that refrigerant and oil are run short because the refrigerant and oil permeate inside the hollow portion through the fine holes. Particularly, the conventional hollow piston manufacturing method has a problem in that maintenance and manufacturing costs are increased as the welding area of the piston must be kept in a high vacuum condition for the electron beam welding.
To solve the above problems, European Patent Publication No. 0,959,227 discloses a friction welding method in which after the first and second piston members are molded, parallel joint surfaces are respectively formed on the front surfaces of the first and second piston members, and then, a hollow piston part is formed by frictional heat generated by the first and second piston members to each other under a state where the two joint surfaces face to each other in an atmospheric condition.
However, under the above conventional method, the welding temperature is not uniformly distributed on the joint surface due to a minute difference in flatness of the joint surface as the joint surface is flat, and so, it is difficult to uniformly keep the welding strength of the joint surface. Additionally, the temperature and the welding strength are gradually increased toward the outer circumference of the pistons due to a difference in angular velocity on the joint surface of the first and second piston members. However, the welding strength of the remaining portions becomes relatively lower as the outer circumferential layer is removed by mechanical processing after the welding.
Meanwhile, U.S. Pat. No. 5,460,317 discloses a friction stir butt welding method, which is different from the above friction welding method. In U.S. Pat. No. 5,460,317, the surface of a continuous or substantially continuous material is provided with a probe of material, which is harder than that of the above material. After the above material and the probe are contacted to each other, the probe is inserted into the material, and at the same time, relatively periodic motion occurs between the probe and the material. A plasticised region is formed in the material around the probe by frictional heat generated by the periodic motion. Then, the material is bonded by coagulating the material.
Such a friction stir butt welding method has an advantage in that it has little defect, such as deformation due to thermal distortion, without restriction in kinds of material. However, the friction stir butt welding method has a disadvantage in that a hole is formed in the friction portion between the materials by the probe after the friction welding.
As shown in FIG. 8, in manufacturing a piston 1, the friction stir butt welding method has a disadvantage in that it is impossible to form coating film of uniform thickness during a coating process after the friction welding and durability of the welded portion is deteriorated because a hole 5 is formed in a friction portion, i.e., welded portion, between the first piston member 3 having a bridge 2 and the second piston member 4.
That is, as the second piston member 4 of the piston 1 used for the swash plate type compressor with variable capacity is rubbed against the inner surfaces of the cylinder bores during reciprocating motion of the piston 1, for abrasion resistance and sealability of the outer circumference of the second piston member 4, a uniform coating film is formed on the outer circumference of the second piston member 4 including the welded portions, and then, an abrasion process is performed as a post-process. Therefore, as described above, if the hole is formed in the welded portion between the piston members, coating liquid penetrates into the hole. When plastic working is performed in this condition, an elevated portion is formed at the position of the hole 5, and so, the coating film is not uniform. Therefore, an abrasion process can be performed eccentrically, thereby resulting in lowering of durability.
The friction stir butt welding method has other disadvantages in that there is possibility to cause bending deformation on the members by pressure when the probe is inserted into the welded portion between the two members, and so, it is difficult to manufacture the piston, which must have a precise outer diameter.
To prevent the bending deformation, Japanese Patent laid-open Publication No. Hei 11-156560 discloses a friction stir welding apparatus of preventing bending of piston members, in which one or more support rollers being in contact with the outer circumferences of piston members are installed at the opposite side of the probe, namely, under the two piston members. The support rollers support pressure generated when the probe is inserted, thereby preventing bending of the piston members.
However, because the support rollers are installed under a state in which they are fixed to support the lower portions of the two members, in order to rotate the two members on the fixed support rollers, the two members must have precise outer diameter to exactly arrange the two members on the central axis line of rotating means.
Therefore, the friction stir welding apparatus of the prior art can manufacture only one kind of piston having the same outer diameter. To manufacture pistons having different outer diameters, it is necessary to change or separately manufacture the welding apparatus according to the outer diameter, thereby increasing the manufacturing costs.